on the line o P, and thus the lead / is unchanged. The angle of
lead is, however, slightly altered from /81 to /32 for the admission
positions of crank must pass through G and H respectively.
P. 666. Lead in Link Motion.—The two diagrams in Fig.
1015 represent the skeleton forms of the Stephenson and Gooch
link motions when the engine crank is at dead-point Taking
the Stephenson gear, the eccentrics are at E E, and the full-gear
positions of the link L L are shewn dotted, bringing the valve eye
to the position F, and the valve v to F, thus open to the smallest
value of the lead. Moving now the link to raised position, shewn
by full lines, the valve eye is displaced into the position M and the
valve to M, thus opening to the largest value of lead. The linkage
forms a four-bar chain rotating round the two centres EE, which
explains the lead variation, the latter being found for any other
position of the link by the same principle of trial. In the Gooch
motion the link is never raised or lowered, and as the gear is only